In the drilling and completion of offshore wells, it is common to employ large diameter pipes extending from a point at or near the floor of the ocean or other body of water to the vessel or platform from which operations are carried out. Risers, caissons or "extended casings" are typical. Particularly when the pipe contains drilling mud or other fluid, the great weights involved place the riser or the like under large compression loads, and it is common practice to tension the pipe to compensate for such loads. In early installations, mechanical devices were employed to apply the desired tensioning force. Subsequently, prior-art workers have employed buoyancy devices, either to reduce the load on the mechanical tensioning device or to eliminate the need for that device. The buoyancy devices have taken the form of hollow metal buoyancy chambers, as shown for example in U.S. Pats. No. 3,017,934, issued Jan. 23, 1962, to A. D. Rhodes et al. and 3,933,108, issued Jan. 20, 1976, to B. G. Baugh, or have been formed from "syntactic foam" materials, such as those disclosed in U.S. Pat. No. 3,522,437, issued Nov. 23, 1971, to E. C. Hobaica et al.
The use of buoyancy devices has attained considerable commercial success but still presents substantial disadvantages. The metal chambers are cumbersome, expensive to construct, have the disadvantage that the metal employed adds substantial additional weight, and require careful design and construction to assure that the sealed nature of the chambers will be preserved during use. The syntactic foam materials are also expensive, and they present substantial production difficulties and make it difficult to achieve quality control and attain high resistance to impact. There has accordingly been a continuing need to achieve buoyancy control in such underwater pipe installations as risers and the like without use of metal chambers or syntactic foam.